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<br />It is common to characterize silts using both drained
<br />and undrained shear strengths similar to clays. For
<br />drained conditions, the shear strength of silt can be
<br />characterized by an effective stress friction angle (φ’)
<br />with an assumed effective stress cohesion (c’) equal to
<br />zero. For undrained conditions, the shear strength of
<br />silts can be expressed using total stress strength
<br />parameters (c, φ) or in terms of undrained strength, Su.
<br />Similar to clays, there are different forms of
<br />characterization that can be used for Su – for example,
<br />constant Su, Su as a function of effective confining
<br />stress, and Su as a function of depth.
<br />Laboratory tests used to measure values of φ’ for silts
<br />include the CD or CU’ triaxial shear test and the DS
<br />test. Laboratory tests used to measure the undrained
<br />shear strength (c, φ or Su) of silts include the UC test
<br />(Su), UU or CU/CU’ triaxial shear test (c, φ or Su), or the
<br />DSS test (Su). Similar to coarse-grained soils, it can be
<br />difficult to obtain quality undisturbed samples of silts
<br />in the field, particularly non-plastic or very low
<br />plasticity silts.
<br />Strength behaviors of silts have not been as widely
<br />studied as those of sands or clays. As a result of this
<br />general lack of research and compilation of data, very
<br />few empirical correlations exist for predicting shear
<br />strength values for silts. Empirical strength correlations
<br />that are available for silts are often regionally specific
<br />and developed with relatively limited data sets. Similar
<br />to sands and clays, SPT, CPT, and shear wave field tests
<br />can be used for empirical strength correlations of silts.
<br />Empirical correlations using results of field tests for
<br />sands can generally be applied to estimate shear
<br />strengths of non-plastic silts. Shear strengths of plastic
<br />silts can generally be estimated from empirical
<br />correlations using results of field tests for clays. It
<br />would be prudent to incorporate a level of
<br />conservatism when using correlations for silts.
<br />Shear Strengths by Federal Agency
<br />Various government agencies have identified shear
<br />strength envelopes to be used for design loading
<br />conditions. Therefore, stability analyses performed for
<br />these agencies should utilize their specific strength
<br />characterization criteria. Reference [1] provides a
<br />useful summary of then current guidance provided by
<br />various federal agencies such as: United States Army
<br />Corps of Engineers; Bureau of Reclamation; United
<br />States Department of Agriculture, National Resources
<br />Conservation Service; and Federal Energy Regulatory
<br />Commission. Guidance documents for each agency
<br />should be referenced for any updates.
<br />Conclusion
<br />Slope stability analyses of embankment dams are
<br />highly dependent on shear strength parameters
<br />assigned to the embankment and foundation soils. It is
<br />therefore very important to perform a detailed shear
<br />strength characterization of the various soils to obtain
<br />meaningful slope stability results. Shear strength
<br />characterization requires both knowledge and
<br />experience to select and develop appropriate
<br />parameters for various embankment and foundation
<br />soils. Shear strengths of soils vary depending on the
<br />loading conditions needing to be analyzed, and the
<br />variations have a significant impact on slope stability
<br />calculations. Careful shear strength characterization is
<br />therefore an imperative, in most cases the most
<br />important, component of slope stability analyses for
<br />embankment dams.
<br />Useful References
<br />[1] Strength of Materials for Embankment Dams, United States Society on
<br />Dams, February 2007.
<br />[2] Soil Strength and Slope Stability, J. Michael Duncan, Stephen G.
<br />Wright, and Thomas L. Brandon, 2nd Edition, 2014.
<br />[3] Slope Stability during Rapid Drawdown, James M. Duncan, Stephen G.
<br />Wright, and Kai S. Wong, 1992.
<br />[4] Rapid Drawdown Analysis – What is An Analyst to Do? John W. France
<br />and Christina J.C. Winckler, 2013.
<br />[5] Fundamentals of Soil Behavior, James K. Mitchell and Kenichi Soga, 3rd
<br />Edition, 2005.
<br />[6] Engineering Field Manual, Chapter 4 - Elementary Soil Engineering,
<br />U.S. Department of Agriculture, Natural Resources Conservation
<br />Service (NRCS), July 1984 (Fourth Printing)
<br />[7] Earth Manual, Part 1, U.S. Department of the Interior, Bureau of
<br />Reclamation, 1998 (Third Edition)
<br />[8] EM 1110-1-1804, Geotechnical Investigations, U.S. Army Corps of
<br />Engineers, January 2001
<br />[9] EM 1110-1-1802, Geophysical Exploration for Engineering and
<br />Environmental Investigations, Department of the Army, U.S. Army
<br />Corps of Engineers, 31 August 1995
<br />[10] FHWA NHI-06-088, Soils and Foundations, Reference Manual –
<br />Volume 1, U.S. Department of Transportation, Federal Highway
<br />Administration, December 2006
<br />[11] Manual on Estimating Soil Properties for Foundation Design, EPRI EL-
<br />6800, Electric Power Research Institute (EPRI), August 1990.
<br />[12] New design procedure for stability of soft clays, Ladd, CC. and Foot, R.
<br />(1974). ASCE Journal of the Geotechnical Engineering Division. Vol
<br />100, No GT7, pp 763-786.
<br />[13] Stress-deformation and strength characteristics. Ladd, CC, Foot, R,
<br />Ishihara, K, Schlosser, F and Poulos, HG. (1977).International
<br />conference on soil mechanics and foundation engineering, 9,
<br />Proceedings, Vol. 2, pp 421- 494. Tokyo.
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